The present disclosure relates generally to Radio Frequency Identification (RFID). In passive RFID, an RFID reader interrogates a passive tag using a transmitter that emits a carrier signal which the tag modulates by varying its reflection coefficient. Because of limited coupler directivity, antenna mismatch, and reflections from the environment, a portion of the transmitted signal (aka self-blocker) enters a receiver in the RFID reader. Therefore, the noise on the transmitted signal can degrade receiver sensitivity and hence the range of the RFID reader. Phase noise on the blocker signal is more tolerable than amplitude modulation (AM) noise as long as this phase noise is correlated with that of the receiver local oscillator (LO) signal. Designs take advantage of this fact by utilizing the same synthesizer for receiver and transmitter so that the higher noise of a fully integrated synthesizer can be tolerated. However, there is no rejection to AM noise, and to reduce it to an acceptable level a very high signal-to-noise ratio must be created and then maintained throughout the transmitter chain. The earlier stages in the transmitter gain lineup can often dominate the total noise. As a result, the digital-to-analog (D/A) converters, baseband gain, low-pass filtering, and modulator stages must be allocated a high current drain. It is also technically challenging to achieve required signal-to-noise levels.
In reader-to-reader interference scenarios, the phase noise of the transmitter and the receiver LO are not correlated so there is no phase noise rejection at the receiver. As a result, the transmitter phase noise will typically dominate over the AM noise as a source of reader-to-reader interference, particularly for nearby channels. Hence, the main problem with transmitter AM noise is its impact on the co-located receiver, especially because the self-blocker is always present at some level. Therefore, mitigating the effect of transmitter AM noise on the reader's own receiver would address the primary concern with AM noise.
Conventional methods for cancellation of the blocker signal are known and can be effective in reducing the impact of the blocker's noise and carrier on the receiver. However, cancellers add sufficient cost and complexity that they're typically only used for higher end products. Also, to be fully effective cancellers must track any changing signal reflections from the environment which can be challenging.
Accordingly, there is a need for an apparatus and method for compensation of AM noise in RFID devices through modulation of a received signal.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.